The present invention relates to apparatus and techniques for the characterisation of fingerprints. The invention has particular, although not exclusive relevance, to such techniques for use in access control systems, in which the fingerprint of a person seeking access to, for example, premises or a bank account, is compared with recorded data identifying the fingerprint of an authorised person.
The invention is concerned with techniques in which relative movement of finger print patterns is used for recognition purposes and typical examples of techniques of this kind hitherto are described in U.S. Pat. No. 4,414,684 and GB Patent Number 1,506,611.
The former patent specification discloses a fingerprint characterisation technique which uses a photodetector to detect coincidence between a recorded optical representation of a fingerprint and the moving projection of an actual fingerprint which is caused to scan in x and y directions through the recorded optical representation on to the photodetector.
The latter patent specification discloses an automatic opto-mechanical image comparator which utilises a similar technique to determine whether a pair of images of a fingerprint are derived from the same fingerprint. In this latter system, when an identical pair of fingerprint images is used, light parts of one of the images coincide directly with the light parts of the other image (similarly for dark parts) when the two images are directly superposed and mutually aligned, resulting in a maximum value of transmission or reflection of light through or from the superposed pair of images. Alternatively, if one of the images is "polarity-reversed" (e.g. a negative of the image is used, rather than a positive), then a minimum value requirement of such a system would be the ability to tolerate unavoidable minor differences between a preferred print and its previously recorded image, caused, for example, by scars or dirt on a finger, or by plastic deformation of the skin during acquisition of the recorded image or the preferred image for matching.
Apparatus and fingerprint characterisation techniques for use in an access control systems which is both reliable and inexpensive is an attractive proposition. It is therefore an object of the present invention to provide apparatus for characterising fingerprints and, in particular, apparatus which is suitable for use in an access control system.
According to the present invention there is provided apparatus for characterising fingerprints comprising: imaging means for providing an image of a fingerprint and superimposing thereon a further image of the same fingerprint;
generating means for generating an autocorrelation pattern of said images by disturbing said superimposition thereby to provide light intensity variations of the pattern;
sampling means for sampling, in each of a plurality of sections of said pattern, the light intensity variations and providing data representative of said variations;
storing means for storing the data;
and retrieval means for retrieving the data for comparison with data representation of at least one further fingerprint autocorrelation pattern so as to determine whether the autocorrelation patterns are derived from the same fingerprints or set of fingerprints.
An autocorrelation pattern is generated when two identical images are superimposed. The autocorrelation pattern is dependent not only on the nature of the original image, but also on the relative spatial positioning of the two images as defined by (a) the modulus, and (b) argument of the vector between a common reference point on each image; and (c) the relative angular displacement (orientation) between the two images.
Preferably the displacement distance (modulus of the displacement vector) of the fingerprint images for generation of the autocorrelation pattern is approximately one-half of the mean inter-ridge spacing of the fingerprint pattern. This is desirable so as to achieve maximum contrast in the autocorrelation pattern produced.
The autocorrelation pattern of the fingerprint may be conveniently divided into quadrants, the light intensity profiles generated by the autocorrelation process within each quadrant being measured as the autocorrelation displacement vector argument is caused to vary. The data so generated may be displayed in graphical form if desired. A quadrant photodiode may be conveniently employed to measure the light intensity profiles.
The apparatus may suitably be used for the purpose of access control.